1. Technical Field
The field of this invention is related to wireless communication, in particular to improving downlink (DL) coverage while deploying unplanned Femto cells (also known as CSG femtocells).
2. Discussion of Related Art
Femto cells (HNB) are expected to be deployed without radio planning by the end users, using either a dedicated radio channel or using the same radio channel the existing planned network (e.g. MACRO NB) uses.
One of the major concerns of Femto cell deployment is how to guarantee the planned network quality of service while deploying the new HNBs. Following the analysis in 3GPP 25.967 V2.0 and 3GPP 25.820 V820 we focus on HNB interference to MUE reception in the DL. More specifically we will focus our description for the more challenging shared frequency deployment (thought the invention is related to dedicated frequency deployment too).
The interference scenario is the one in which a UE detect a HNB with better signal but is banned from using this HNB and must use existing MNB that has lower signal level. The ban could be due to the used HNB UE access list (e.g. CSG) or due to the HNB limits (e.g. it reached the maximal number of supported UE).
For most MNB connected UEs HNB power will be treated as part of the total noise level (some exception could be UE with interference cancelation). The increase of the noise level could cause coverage holes to the MNB deployed network. It is clear that the severity and the size of those coverage holes (dead zones) depend on the HNB transmitted power, so the main mitigation is to reduce the HNB power.
In the literature there are several approaches to set the HNB Tx power, those approaches may be static in nature (i.e. fixed power), based on MUE measurements relayed to the HNB or its controlling entity via serving MNB or based on the HNB measurement of the existing network during the sniffing period. Several examples to such schemes appears in the following documents, which are incorporated herein by reference in their entirety: U.S. Patent Publication No. 2009/0042594 which discloses adaptation of transmit power based on maximum received signal strength; U.S. Patent Publication No. 2009/0042595 which discloses autonomous adaptation of transmit power; U.S. Patent Publication No. 2009/0042596 which discloses adaptation of transmit power based on channel quality; 3GPP 25.967 V2.0; 3GPP 25.820 V820; and R4-091351, NTT DOCOMO, Enhanced HNB interference coordination based on network control.
Each of those technique suffer from drawbacks, the main drawbacks are:                Fixed power could not adapt to all deployment scenarios        MUE measurement: There is a need to transfer the interference indication to the HNB or to its controlling entity. The information transfer requires modifying the existing network        For sniffing based measurement: The path loss between the MNB and HNB could differ from the path loss between the MNB and target MUE. For example the HNB has line of sight to the MNB while the MUE does not        
AbbreviationsCSGClosed Subscriber Group. Only specific UE may be served by the HNB.DLDownlink, the RF path from BS to UEHNBHome NodeB, also known as Femto cell.HUEUE camping on HNB cellMNBMacro NodeB. We will use this definition to related to all planed NB, including Micro and Pico cells.MUEUE camping on Macro cellNBNodeBOSGOpen Subscriber Group. All UE within network may connect to the HNB.PLPath LossQOSQuality of service.RX, RxReceive, ReceiverSIBSystem Information BlockTX, TxTransmit, TransmitterUEUser Equipment, also cellular terminalULUplink, the RF path from UE to BS
Other terminology the one used in 3GPP specification, and are well known.